There is an emerging appreciation that heme proteins such as myoglobin and hemoglobin catalyze the metabolism of nitric oxide (NO) and nitrite, and thereby modulate cell and tissue responses to hypoxia. Over the last five years our group has published biochemical and physiological studies that suggest a novel function for hemoglobin as a nitrite reductase that generates NO under physiological and pathological hypoxia, suggesting that myoglobin, the other major heme globin, may also possess a capability to mediate the metabolism of oxides of nitrogen in a manner influenced by oxygen tension. Analogous to the bacterial nitrite reductases, a concerted proton and electron transfer reaction to nitrite reduces the anion to NO. We have also discovered a novel nitrite anhydrase activity that converts two nitrite molecules into the highly diffusible, nitrosating molecule N2O3, allowing efficient NO signaling in a heme rich environment. In the current research proposal, the NO/ N2O3 signaling pathways will be explored in myoglobin, with special focus on an ability to regulate cellular responses to hypoxic and ischemic stress by a) modulating cellular metabolism via the regulation of mitochondrial electron transfer reactions and b) activating net cytoprotective cell signaling reactions after ischemia-reperfusion injury. These concepts will be addressed by testing the hypothesis that myoglobin-mediated nitrite metabolism regulates hypoxic NO signaling and promotes NO bioavailability in the heart. More specifically, using mutant myoglobin proteins in in vitro biochemical and cell culture systems as well as an in vivo model of myocardial infarction, we aim 1) to determine the molecular and enzymatic mechanisms underlying the nitrite reductase activity of myoglobin in hypoxia, 2) define critical molecular targets of myoglobin-derived NO and N2O3, and 3) to determine the role of myoglobin in regulating downstream cytoprotective signaling (apoptosis and mitochondrial biogenesis) after ischemia/reperfusion. Successful completion of the proposed research plan will advance our understanding of the biological function of myoglobin and the physiological and pharmacological potential of nitrite in the cardiovascular system.